Kc green lantern + force field + shellhead
Can I use shellhead on kc green lantern equipped with force field?

No. Figures must meet the prerequisits for the feat before the game begins (during the team building). Even though Force Field grant Toughness to GL, he doesn't gain that Toughness until the game begins. And since he doesn't have Toughness duringt he team building phase, he can't be assigned Shellhead.